Most fossil fuels, and coal almost invariably, contain sulfur contaminants. When these are burned, such as in the fluidized-bed combustion process, the fossil fuels are converted in an oxidized atmosphere to a hot flue gas containing gaseous sulfur contaminants, predominantly in the form of sulfur dioxide. In order to render this flue gas environmentally suitable, the sulfur contaminants of the gas must be reduced by desulfurization.
Evaluation of the fluidized-Bed Combustion Process (EPA Report 650/2-73/048; pb 231 163) describes a typical process for preventing the sulfur contaminants from being released into the flue gas during the conversion of the fossil fuel into the gas. The process employs a fluidized-bed of calcium oxide containing material which acts as a sorbent for the sulfur contaminants by capturing the sulfur contaminants in the form of calcium sulfate. This process may be operated regeneratively or non-regeneratively depending on environmental and economic factors.
The poor efficiency of the usual calcium oxide containing materials to remove sulfur impurities from gases is illustrated by the Moss et al. U.S. Pat. No. 3,617,583 where the sulfur content of the lime used to remove the sulfur gases, in the fuel gasifier, is only 2.4%. It is of considerable significance, therefore, to increase the efficiency of this type of sulfur sorption, particularly in view of the present energy crisis. Thus, the United States has large deposits of sulfur-containing coal which could be put to use to reduce the dependence of the United States on foreign oil, if there were more efficient ways to remove sulfur contamination from the gases evolved by the combustion of such coal.
In the above process the calcium oxide containing materials that are utilized to capture the sulfur contaminants are usually derived from either limestone or dolomite; several methods of calcination have been proposed to achieve this conversion. The standard process is described by Kirk-Othmer, The Encyclopedia of Chemical Technology, "Lime and Limestone," Vol. 12, 1967, page 434 and also discussed in U.S. Pat. No. 2,408,647. Kirk-Othmer teaches calcining the limestone or dolomite in a single stage by heating the stones in compliance with the following three essential guides: (1) the stones must be heated to the dissociation temperature of the carbonates; (2) this minimum temperature (but practically, a higher temperature) must be maintained for a certain duration; and (3) the carbon dioxide gas that is evolved must be rapidly removed.
There have also been proposals for multistage calcination processes. U.S. Pat. No. 2,228,618 teaches a two-stage heating process where the temperature of the first stage is lower than that of the second stage and the carbon dioxide generated in both stages is immediately vented from the system.
U.S. Pat. Nos. 3,483,280 and 2,370,281 issued to Einstein et al. and Azbe respectively, propose methods of calcining either limestone or dolomite by multistage heating where generated carbon dioxide is present in the initial burning chamber. Both patents utilize either the carbon dioxide that is obtained by recycling or that is generated in subsequent heating stages to furnish the burning chamber with a substantial amount of optional additional heat, which would otherwise be furnished by greater quantities of fuel gas. Azbe (page 4, col. 2, 11 31-37) cautions against the dangers of recarbonation, i.e. absorption of CO.sub.2.
An article by Azbe given on May 28, 1925 (see Theory and Practice of Lime Manufacture, pp. 24-26 entitled "Part IV--Judging Kiln Peformance by Gas Analysis") describes good conditions for calcination as shown in chart 3 of FIG. 9 (the far right-hand chart) wherein a CO.sub.2 volume percent of a little less than 30% to a little less than 35% is suggested for a batch-fired process, the CO.sub.2 dropping to 10% (end of third paragraph, col. 2, page 26).
U.S. Pat. No. 948,045 to Floyd discloses a method of reacting limestone with carbon to produce carbon monoxide and quicklime, i.e. calcined limestone predominantly calcium oxide or hydroxide. The patentees emphasize the absence of CO.sub.2 gas pressure in the retorts (pg. 4, 1. 23).
Wicke et al. U.S. Pat. No. 3,991,172 discloses the manufacture of what is referred to as "reactive calcium oxide" by decomposing very small particles of calcium carbonate at a temperature of at least 850.degree. C. and under an atmosphere wherein the CO.sub.2 partial pressure is not greater than 40%, and most preferably no more than 20%, of the equilibrium partial pressure. The patentees state that "the CO.sub.2 partial pressure of the atmosphere must be ... considerably below the equilibrium pressure of the system CaCO.sub.3 - reactive CaO." (col. 2, 11. 56-61)
EPA Report 650/2 - 74-001 (January 1974) pp. 59-63 is another example showing conventional calcination of limestone at low CO.sub.2 partial pressure. This publication indicates the off-gas concentration of CO.sub.2 to have been 21%.
The literature supports the following general conclusions:
(1) Higher temperatures of calcination produce more dense, less reactive CaO.
(2) Higher heating rates before and during calcination produce stones of lower porosity and less reactivity.
(3) Long retention times (heating after complete decomposition of CaCO.sub.3), always cause sintering, in air or vacuum.
(4) Impurities increase the sintering of the stone, because of the lower Tammann temperatures which permit diffusion into line cracks and fissures.
(5) Sodium, unlike other impurities, may increase lime reactivity by decreasing shrinkage particularly in stones prone to high shrinkage.
(6) There is disagreement as to the relative importance of temperature and heating rate on lime reactivity. Temperature is probably the more important factor until heating rates get extremely high.
(7) Calcination atmosphere has been shown to affect the structure of CaO. Thus, according to Beruto et al. (Nature, Vol. 263, 1976, pp. 221-222) decomposition in vacuo rather than in N.sub.2 or air produces a calcine with high internal surface area.
From a review of the prior art it is further apparent that the requirement of using low quantities of CO.sub.2 in the calcining atmosphere has been universally followed. Normally the quantity of CO.sub.2 has been reduced as much as possible because CO.sub.2 in this atmosphere retards calcination and slows down the process, in addition to potentially causing the problems mentioned by Azbe in U.S. Pat. No. 2,370,281.